Many disposable sheet goods such as toilet tissue, paper towels, gift wrap, aluminum foil and the like, are sold in the form of a roll supported by a tubular paperboard core. Because of the strength required in the paperboard core during the process of winding the disposable sheet goods onto the core, the core is normally formed of at least two radially superposed layers, which in turn, are formed from separate spirally wound paperboard plies. Each of the spirally wound paperboard plies forms a helical seam which extends in the axial direction along the paperboard tube and which results from abutment of the opposed longitudinally extending edges of the ply along the length of the tube. During the tube manufacturing process, the separate paperboard plies used to form radially adjacent tube layers are positioned with their respective edges axially offset from each other as they are wound onto a mandrel so that the seams formed by the respective separate layers are displaced from each other in the direction of the tube axis. In other words, the helical seams of the adjacent layers do not overlap.
The paperboard tube making process is conducted by winding the innermost paperboard layer onto a stationary mandrel while simultaneously winding one or more exterior paperboard plies successively radially outwardly from the exterior of the first ply. An adhesive coating is applied to the exterior face of the inside paperboard ply and/or to the interior face of the adjacent exterior paperboard ply. As a result, radially adjacent plies forming separate layers adhere strongly to each other so that the tube can have considerable strength. Although each of the spirally wound layers includes a continuous helical seam, the composite tube formed from several layers does not readily unravel because the seams in adjacent paperboard layers are offset radially from each other as mentioned above, and because of the substantial surface bonding between adjacent tube layers.
Particularly in those cases where the paperboard tube is used as a core support for a disposable sheet material such as paper towels, toilet tissue, or the like, it is highly desirable to minimize the cost of the paperboard core. This has been achieved in typical commercial practice by minimizing the number of layers of paperboard used to form the core and by minimizing the cost associated with the paperboard forming each of the layers. Accordingly, commercially available cores are preferably formed from only two layers and each layer is formed from a relatively inexpensive and weak paperboard, typically of relatively low density and having a high content of recycled material.
As will be apparent, there is a limit to the minimum strength of paperboards that can be used to manufacture paperboard cores. Thus, the cores cannot be made from materials which are so thin and/or weak that they will not form a self-supporting structure upon being wound into helical form because the tube structure must provide support to the sheet material which is wound onto the core. Similarly, the paperboard tube must be formed from at least one layer, and in commercial practice, at least two paperboard layers are used because of the substantial strength resulting from the bonding and proper alignment of the multiple layers.
Various attempts have been made to make paperboard tubes from a single layer of paperboard by forming an overlap joint along the helical seam. Thus, attempts have been made to overlap one edge of the ply onto the top of the other edge of the ply as the ply is wound onto the mandrel. However, these attempts have not resulted in production of a commercial paperboard core product when a relatively weak, low basis weight material is used because of various difficulties. For example, it is difficult to properly bond the overlapped joint; however, improper bonding results in tube having a single continuous helical seam which is apt to unravel.
Another problem associated with overlapped joints is the uneven exterior and interior surface which normally results. The tube is thicker in the overlapping joint area and thus includes a raised helical seam extending along the exterior of the tube surface. Similarly, a corresponding inside surface of the tube can also be uneven; in other words, the inside surface of such tubes can also include a helical raised region extending from end to end of the tube. The uneven inside surface can be problematic for inserting the tube onto a winding mandrel and/or removing the tube from the mandrel during the process of forming a roll of sheet goods thereon. Similarly, the exterior uneven surface can be problematic as it can impact negatively on the appearance on materials wound onto the tube, particularly materials such as foils and wrapping paper.
In order to eliminate the raised regions associated with overlapped helical joints, paperboard plies having edge portions which are thinner than the middle portion of the ply have been used in an attempt to form an acceptable single ply tube having an overlapped edge seam of a thickness substantially the same as the non-overlapped portions of the tube wall. However, in practice, the costs associated with forming the thinner edges of the paperboard plies can substantially increase the cost of the ply because the process used to reduce edge thickness must be carefully controlled and also increases the overall manufacturing time required to produce each tube. Thus, the process of forming the thin area on the edges of the paperboard ply must desirably result in an edge having an uniform thickness. Particularly when the paperboard is relatively inexpensive and thus, relatively weak, substantial efforts are required not to overly deform the edges, while at the same time deforming the edges sufficiently to achieve the desired degree of thickness reduction. In practice, edges of plies have been treated to decrease their thickness for the formation of paperboard tubes, by a grinding or compressing process in which the edges of the paperboard ply are ground with an abrasive wheel, or compressed between compressing rollers to decrease their thickness. However, as indicated above, costs associated with such treatments substantially increase the costs of the paperboard plies and these treatments are quite difficult when conducted on low basis weight paperboard.
Although paperboard plies having compressed edges have been used by the assignee of the present application to form a single layer paperboard tube, the paperboard used in this process has been a relatively high strength, high density paperboard, having a basis weight above 76 lbs/1000 sq. ft., and a thickness of about 0.025 inch. The paperboard plies used to form these tubes have also been treated in a deckling process in order to reduce the thickness of the longitudinal edges of the plies. This treatment is conducted after the ply has been cut from considerably wider paper sheet, and the deckling process has been conducted to provide longitudinal edges which are compressed on opposing faces of the ply to improve the lay up of the overlapped joint during the winding process.
The process of forming such deckeled edges on paperboard plies is rather expensive because each ply must be treated separately. Moreover, the costs associated with the relatively high basis weight paperboard adds to the expense involved in forming the tubes. These prior single layer tubes have been used to support relatively expensive gift wrapping papers in which the importance of the appearance of the product justifies a higher cost. However, the costs associated with such paperboard is generally too high for use in the production of core support tubes for less expensive products such as toilet paper, paper towels, etc.
It has also been found in practice that a uniform and properly bonded overlapped joint is particularly difficult to achieve when attempts are made to form single wall tubes from relatively low basis weight, relatively weak paperboard plies. When an overlapped joint is formed, substantial tension must be applied to the tube-forming ply during the spiral winding process. This is necessary so that the overlapping edge will make substantial and uniform contact along the length of the tube. At the same time, the primary portion of the paperboard layer must make substantial contact with the supporting mandrel to avoid formation of an uneven interior surface. If either of these conditions are not met, the paperboard tube can have an uneven, wrinkled appearance and/or will not be uniformly bonded along the overlapping joint. However, with low basis weight, weak paperboard plies, the tension which must be applied to the plies during the winding process in order to achieve the necessary degree of contact can result in the tearing of the paperboard plies, which in turn, results in shutting down of the tube making process. Although adhesion of paperboard materials can be increased by using increased amounts of adhesive, this is particularly difficult in producing a single ply tube because the application of an overabundance of adhesive can cause portions of the adhesive to flow within the joint and to leak from the joint onto the winding mandrel with the result that tubes can not be formed on the mandrel until the process is stopped and the mandrel is cleaned.